FIG. 1 shows a basic architecture of an Internet Protocol (IP) Multimedia Subsystem (IMS).
Devices allowing end users to access network services are called User Equipments (UE). UEs access the IMS network to be able to use services provided by the IMS network. UEs communicate with the IMS network by attaching to a packet switched (PS) network, such as General Packet Radio Service (GPRS) network. The UE may access the IMS network over a radio access network (RAN) or over a fixed line access network, such as Digital Subscriber Line (DSL) access.
The IMS network comprises Session Initiation Protocol (SIP) proxies which process SIP signaling. SIP is an application-layer control protocol for creating, modifying, and terminating sessions with one or more participants. SIP proxies in the IMS network are called Call State Control Functions (CSCF) and the main features of the CSCF are to perform session control functions, service control functions and address translation functions. Depending on the functionality the CSCF provides, it is called Proxy-CSCF (P-CSCF), Interrogating-CSCF (I-CSCF) or Serving-CSCF (S-CSCF).
For a UE the P-CSCF (not shown) is the first contact point in the IMS network. When the UE registers to the IMS network, a P-CSCF is allocated to the UE. The same P-CSCF remains allocated to the UE during the lifetime of the registration, and all signaling between the UE and the IMS network traverses the P-CSCF. The I-CSCF (not shown) is located at the edge of the administrative (operator) domain, in signaling path between the P-CSCF and the S-CSCF. The I-CSCF routes received requests to a correct destination, typically to an S-CSCF. The S-CSCF implements the session control in the IMS network and acts as SIP registrar. All SIP signaling initiated by or destined to a certain UE traverses the S-CSCF assigned to serve the UE.
A Home Subscriber Server (HSS) or User Mobility Server (UMS) stores all the user related subscription data, such as location information, security information and user profile information defining the services that the user is subscribed to. Subscription data is created into the HSS when an end user obtains an IMS subscription from an IMS service provider. The subscription data is associated with an IMS private user identity (IMPI) of the user. The HSS also maintains information about the S-CSCF allocated to the user. During an IMS registration procedure, the I-CSCF may contact the HSS in order to find out the address of the allocated S-CSCF based on the IMS public user identity (IMPU) and IMS private user identity (IMPI) of the user sending the registration request. Once the correct S-CSCF has been reached, the S-CSCF contacts the HSS over Cx interface to download user profile information that includes a service profile for the user.
Application Servers (AS) may be part of the IMS network or alternatively are so-called third party ASs located outside the IMS network. ASs host and execute services using SIP and may have an interface, called Sh interface, to the HSS to retrieve user specific data needed in service execution. The S-CSCF may forward a SIP request to an AS over an IMS Service Control (ISC) interface. The S-CSCF obtains information about ASs from the above mentioned service profile included in the user profile downloaded from the HSS.
For establishing a session between an inviting UE and an invited UE, an S-CSCF forwards SIP requests towards the invited UE, possible after first giving the session control to an AS for executing a service and after the AS has given the session control back to the S-CSCF. Instead of the invited UE, the request could be terminated to an AS, for example to a voice mail server.
FIG. 2 shows a structure of Filter Criteria that is basically a data structure including user specific service information from which an S-CSCF is able determine when an AS is to be contacted for providing a service to the user. User profiles are stored, in the HSS, per Private User Identity (IMPI) and each user profile may comprise several service profiles associated with a Public User Identity (IMPU). Filter Criteria are included in the service profile of the user profile downloaded from the HSS to the S-CSCF during the IMS registration procedure.
Filter Criteria include a priority field. If the service profile comprises more than one Filter Criteria, the S-CSCF assesses Filter Criteria according to the priority of each Filter Criteria (highest priority is taken first). A Trigger Point comprises one or more service trigger points which define the exact conditions of which have to be fulfilled in order to start a service and a certain AS is to be contacted. Examples of service trigger points are the presence of a specific SIP method (INVITE, OPTIONS, SUBSCRIBE, . . . ), partial or full match on the content of any SIP header or SDP line, presence or absence of any SIP header, and content of Request-URI.
Filter criteria further include the address of an AS (SIP URI) to be contacted if a service is to be started based on fulfilled conditions of one of the service point triggers. A Default Handling parameter of Filter Criteria defines the action to be taken if the AS to be contacted is not available, i.e. cannot be reached. The possible actions in this case are abort or continue the processing of the SIP request. Finally, Filter Criteria could include service information, which is transparent service related information that an S-CSCF may forward to an AS when contacting the AS.
As described above, it may happen that an S-CSCF is not able to contact an AS which is defined in the Filter Criteria to provide the service in question. Possible reasons for unavailability of the AS are, for example, some very fatal problem in the AS or an error in ISC interface configuration. The S-CSCF must then proceed according to Default Handling instructions in the Filter Criteria. However, even if the S-CSCF is able to contact the AS, further problems may occur relating to service execution in the AS. For example, the AS is not necessarily able to provide the service based on the information received from S-CSCF or an AS internal error may occur during the service execution. In these cases, the AS responds to the S-CSCF with a SIP response code indicating that an error has occurred and possible more detailed information about the nature or type of the error. The above mentioned Default Handling parameter neither is examined nor used in these cases, since the AS was actually contacted successfully and the problem occurred only later, during the service execution in the AS.